1. Field of the Invention
The present invention relates to a method for extraction of metals selected from Cr, Mo, Pd, Tc, W, Re and Pu by methyliminobisalkylacetamide.
2. Prior Art
High-level radioactive liquid waste in the field of atomic energy contains metals such as Pd, Tc, Mo, and Pu. Since Tc has a long half-life period, and therefore has strong radioactivity in high-level radioactive liquid waste after interim storage; thus, there remains a potential threat for a long period of time. Mo has a limit of concentration in vitrified waste. (Advanced Nuclear System Research and Development Directorate of Japan Atomic Energy Agency, “Feasibility Study on Commercialized Fast Reactor Cycle Systems Technical Study Report of Phase II, (2) Nuclear Fuel Cycle System”, JAEA-Research 2006-043 (2006)). Pu is an element that is contained in large amount in used uranium fuels, and can be used as fuels in fast breeder reactors. These metals such as Pd, Tc, Mo, and Pu are thus required to be separated and recovered from high-level radioactive liquid waste.
However, since these metals are present in the form of anions, such as TcO4− and MoO4−, in aqueous solutions, it is difficult to extract the metals by a common extractant (e.g., ALIQUAT 336, tetraphenylarsonium salts). Various methods for extraction of Tc have been proposed (“COMPARATIVE SYNERGISTIC (TECHNETIUM-ACTINIDE) EXTRACTION CHEMISTRY BY TRIBUTYLPHOSPHATE AND SOME AMIDE EXTRACTANTS” N. Condamines, C. Musikas and L. H. Delmau, CEA-CON-11456 (1993) and “Solvent Extraction of Tc(VII) and Re(VII) from Nitric Acid Solution with the Mixture of CMPO and TBP” M. Takeuchi, S. Tanaka, M. Yamawaki, Radiochim. Acta 63 (1993) 97-100), but all of the methods are inadequate because distribution ratios are low and Tc is extracted together with uranium. Further, Pd in high-level radioactive liquid waste is distributed into an organic phase in a solvent extraction system. However, when a concentration of Pd in the organic phase increases, a heavy organic phase (third phase) in which Pd are concentrated are formed. This sometimes causes a problem that lines of a mixer-settler equipment are clogged during process operation. Concerning to Cr, W, and Re, they are present in the form of oxoacids (Cr2O72−, WO42−, ReO4−) in aqueous solutions, and it is generally difficult to recover from the solutions. Further, since high-level radioactive liquid waste is a nitric acid aqueous solution, a preferred organic solvent for use in solvent extraction is dodecane, which is stable and has low toxicity, but conventional extractants cannot be dissolved-well in n-dodecane.